A variety of methods for analyzing quality-deteriorated parts of a network have been proposed (see Patent Documents 1 and 2, for example). FIG. 26 is an explanatory diagram showing an example of a network as a target of the quality-deteriorated part analysis. The network includes nodes such as switching nodes (layer 2 switches, layer 3 switches, routers, etc.), bridge nodes and gateways. In the example of FIG. 26, nodes A-O correspond to these nodes. The nodes are connected together via links. The links can be implemented in various styles, such as LAN cables, fiber-optic cables and wireless links. In FIG. 26, directed links 1-44, which are defined taking also their directions into consideration, are shown as an example. In the figure, the direction of each directed link is indicated with an arrow and the reference character (reference numeral) of each directed link is shown beside the link.
An example of the analysis of the quality-deteriorated part in the network illustrated in FIG. 26 employing the technique described in the Patent Document 1 will be explained below. In the case where the quality-deteriorated part in the network of FIG. 26 is analyzed employing the technique of the Patent Document 1, probes a1-a4 for measuring the quality of each flow and a quality analysis server 1000a for analyzing the quality-deteriorated part are used as illustrated in FIG. 27. It should be noted that the probes are referred to as “terminals” and the quality analysis server is referred to as a “quality-deteriorated part estimating server” in the Patent Document 1. The probes communicate with each other, measure communication quality between the probes (hereinafter referred to as “inter-probe quality”), and periodically transmit information representing the inter-probe quality to the quality analysis server 1000a. 
FIG. 28 is a block diagram showing an example of the configuration of the quality analysis server in the technique described in the Patent Document 1. The quality analysis server 1000a includes a quality information gathering unit 1710, a path information gathering unit 1720, a flow link table management unit 1750, a flow link table storage unit 1760 and a quality analysis unit 1770. The quality information gathering unit 1710 receives the information on the inter-probe quality measured by the probes. The path information gathering unit 1720 receives path information (information on paths between probes) determined by another network system and generates a path table by gathering the path information. The path table is a table which describes whether each flow passes through each link or not. The flow link table management unit 1750 generates a flow link table based on the path table and the inter-probe quality which the quality information gathering unit 1710 received from the probes. The flow link table is a table made by adding the quality measurement result of each flow to the path table. The flow link table storage unit 1760 stores the generated flow link table. The quality analysis unit 1770 analyzes the quality-deteriorated part based on the flow link table.
In the technique described in the Patent Document 1 (hereinafter referred to as a “related technique 1”), the paths of the flows used for the analysis are determined by another system in accordance with a general routing method and the quality analysis server 1000a is informed of the thus-determined paths. Since the quality-deteriorated part analysis of a network is conducted using the difference in the path that each flow passes through, detailed analysis is made possible by measuring the quality of a variety of paths. For this purpose, the communication among the probes can be executed by full-mesh communication. The number of flows in the full-mesh communication can be calculated with the following equation (1):The number of flows in full-mesh communication=(the number of probes)×(the number of probes−1)  (1)
In the case of FIG. 27 where four probes are placed at the illustrated locations, the number of flows in the full-mesh communication is 12 according to the equation (1). FIG. 29 shows an example of the path table in this case. In the path table, each column corresponds to each directed link (link number of each directed link) and each row corresponds to each flow. In a row representing a particular flow, a flag “1” is described in each column representing a directed link through which the flow passes. For example, the first row of the table shown in FIG. 29 indicates that the flow from the probe a1 to the probe a2 passes through links 13, 23 and 25. In the example of FIG. 5, among the total of 44 directed links in the network, the number of links through which any one of the flows used for the analysis passes is 12. The communication quality is monitored in the 12 directed links. The number “12” of the links corresponds to 27% of all the links. The ratio of the number of the monitored directed links to the total number of directed links will hereinafter be defined as a “coverage ratio” (coverage ratio=(the number of monitored directed links)/(total number of directed links)).
The number of types of description when the path table is viewed in the column direction (the number of combinations of rows holding the flag “1” counted in the column direction) equals the number of discriminable quality-deteriorated parts. In the path table shown in FIG. 29, the number of description types is “8”. Each description type when the table is viewed in the column direction will hereinafter be referred to as a “sectioning division”. In the example of FIG. 29, the links 19 and 21 have identical descriptions in the column direction. Therefore, it is impossible to judge which one of the links 19 and 21 is the quality-deteriorated part when quality deterioration has occurred in one of the links. In contrast, the links 13 and 14 have different descriptions in the column direction, and thus it is possible to judge which one of the links 13 and 14 is the quality-deteriorated part when quality deterioration has occurred in one of the links. The quality deterioration analysis is impossible for links through which no flow passes.
Also in the technique described in the Patent Document 2, the paths of the flows used for the analysis is determined by another system in accordance with a general routing method and the quality analysis server 1000a is informed of the thus-determined paths. The key feature of the technique of the Patent Document 2 is that it increases the number of types of paths by adding flows. In this technique, a flow including part of the path of a flow already communicated between probes is added by changing the TTL (Time To Live), etc. For example, assuming that the communication among probes has been conducted in the related technique 1 using the flows shown in FIG. 30, if the technique described in the Patent Document 2 (hereinafter referred tows a “related technique 2”) is applied to the environment shown in FIG. 30, two flows are added to the environment as shown in FIG. 31. FIG. 32 shows an example of the path table in this case. In comparison of FIG. 32 with FIG. 29, the number of sectioning divisions has increased to 12 due to the addition of two flows even though the coverage ratio remains at 27%. Therefore, the quality-deteriorated part analysis can be conducted in more detail compared to the case where the path table of FIG. 29 (the number of sectioning divisions is “8”) is used.
Meanwhile, there has been known a communication system in which a controller determines, for each flow, the operation of each node upon reception of packets and makes settings regarding the action of each flow for each node on the path of the flow. Protocols allowing the controller to control the nodes include a protocol called “OpenFlow”. Specifications of OpenFlow are described in Non-Patent Document 1, for example.